1. Field of the Invention
The invention relates to the field of effluent filtration installations, particularly for water, for the purposes of its purification and for making it potable. More precisely, the invention relates to the field of such installations that include filtration membranes that are directly immersed in the effluent to be treated. Yet more precisely, the invention relates to a method of cleaning the membranes in such installations.
2. Description of the Prior Art
Immersed membrane installations are characterised by the use of micro-filtration or ultra-filtration membranes that may be flat, tubular or with hollow fibres, generally grouped together in modules and do not include any housing. These modules are immersed directly in the treatment tank containing the effluent to be filtered, and the permeate is extracted by aspiration. Such filtration installations are notably described in the patent U.S. Pat. No. 5,248,424 in the names of Cote et al. in the European Patent Application EP-A-510328 and in the article entitled "Direct solid-liquid separation using hollow-fiber membrane in an activated sludge aeration tank" in the names of Yamamoto et al. that appeared in 1989 in the journal Water Science Technology, vol. 21, pp. 43-54.
The immersed membranes employed in such installations are usually used under conditions that lead to little clogging at a low trans-membrane pressure that generally does not exceed 0.5 bar so as to space apart the cleaning operations for these membranes as much as possible. However cleaning operations remain necessary and are normally carried out with the help of chemical solutions that are usually hot.
In installations with traditional membranes, in which the filtration modules are not directly immersed in the effluent to be filtered, but are fitted with a housing and equipped with a filtration loop, the cleaning of the membranes can be easily carried out without taking the membranes out of the installation. This type of cleaning, called in-place cleaning, consists simply of circulating a cleaning solution in the recycling loop. Such a method is effective since it allows good control of the concentration of chemical product in the cleaning solution, the temperature of this solution and its contact time with the membranes. Furthermore, such a cleaning procedure can be made completely automatic. Finally, the volume of discharge is low and corresponds to the ullage of the recycling loop.
However, filtration installations of the type with immersed membranes do not incorporate either a housing or a recycling loop. One of the disadvantages associated with the use of such installations is therefore the fact that the cleaning operations are made much more difficult by the absence of such a housing surrounding the filtration modules and also by the absence of such a recirculation loop.
Several methods of cleaning such filtration installations with immersed membranes have been provided in the state of the technology.
One of these methods, called ex-situ cleaning consists simply of taking the filtration modules out of the tank one by one and cleaning them in a device specially provided for this purpose. Such a method allows effective cleaning of the membranes to be carried out but has a number of disadvantages. On the one hand, it causes the shut-down of the installation or a reduction in its efficiency during the relatively long time required to transfer the modules into the cleaning device and to carry out the cleaning operation itself. In addition, such a method also has the disadvantage of being difficult to automate which increases the cost of the method.
It has also been suggested in the state of the technology, to clean filtration installations with immersed membranes by replacing the effluent present in the treatment tank by a cleaning solution and operating the installation in the usual way so as to allow passage of the cleaning solution through the pores of the membranes. Such a technique also has numerous disadvantages. Although it is effective and can be automated, this method, in effect requires the use of a large volume of cleaning solution. Apart from the fact that the cost of the reactants is increased, it is more difficult and also more costly to heat up such a large volume of cleaning solution. Finally, the volume of the discharge (dirty cleaning solution) is also increased.
It should also be noted that, in the state of the technology a method has been proposed that aims at allowing the in situ cleaning of the membranes of an installation that includes such immersed membranes. Such a method, notably described in the American patent U.S. Pat. No. 5,403,479 in the names of Smith et al. consists of circulating a cleaning solution through the membranes along a flow path opposite to the filtration flow and this without emptying the tank within which the membranes are installed. The surplus cleaning solution not passing through the membranes is recycled in such a way as to minimise the volume of solution transferred into said tank
The effectiveness of this method is limited since the cleaning solution used is inevitably diluted by the effluent present in the tank as soon as it has passed through the membrane, which considerably reduces its effectiveness. At the same time, the temperature of this cleaning solution reduces equally abruptly as soon as it has passed through and this also reduces its effectiveness. In addition, the period for injecting the cleaning solution must be limited in time so that the treatment in progress is not disturbed particularly since when it is a biological treatment, the biomass present in the tank can be quickly decimated if the cleaning solution is injected for too long a time. Finally, such a method cannot be employed when the immersed membrane installation under consideration is used within the context of making the water potable since the chemical reactants used in the cleaning solutions are incompatible with such a treatment